Originally posted by jonhock Could anybody enlighten me as to what happens when a TTL flash (pentax) is used on a P-TTL (pentax) camera? I'm fairly sure I understand the difference between the two technologies but I can't quite grasp what the ramifications would be when using this combination.
I've got 4 TTL flashes and I'm concerned about what to do with them if they don't work with a K20 or a K-x.
Basically, nothing happens. A TTL flash simply won't function (at least in TTL mode) on a P-TTL camera. It can only be used in full manual mode, or auto-flash mode, if the flash itself has the necessary built-in light sensor. Most TTL flashes do not have such a sensor, because their design is based on the assumption that the camera does.
A TTL camera has a flash sensor built into the mirror box of the camera. It reads the reflection of light off the sensor during the exposure.
A P-TTL camera has no such sensor in the mirror box. Rather, a pre-flash (the P in P-TTL) is fired while the mirror is still down. The same light sensors in the viewfinder that measure daylight are used to measure the pre-flash and calculate the necessary exposure. The mirror then flips up, the shutter opens and the camera tells the flash how long to flash.
Take a picture with the built-in flash on your camera. Pay attention in the viewfinder. You should be able to see the pre-flash through the viewfinder. If you have a TTL flash, set it in manual mode and take a picture. You won't be able to see the flash through the viewfinder, because the mirror is up.
As for what to do with your old TTL flashes, you can always use them off-camera. Take a look at The Strobist website at
http://www.strobist.blogspot.com/. You will need some optical slave triggers or radio triggers.